iOS Simulator Not Working

I recently downloaded Xcode 4 to my Mac running lion. The first thing I did after successfully downloading xcode was create a single-view application and select "Run > iPad 5.1 Simulator" and then click run. iOS Simulator would open and nothing would appear.

I went to the force quit menu and it said that the iOS Simulator wasn't responding. I restarted Xcode and my computer, but neither worked. I tried creating a new project and it didn't work. I downloaded the newest SDK's and it still didn't work.

Any suggestions???